We compare the resilience of a high-dimensional QKD protocol to noise resulting from classical communications coexisting in the same fiber channel. We find that a 4D protocol makes QKD possible with classical noise levels that are 5 dB higher compared to a 2D protocol.
We investigate the resilience to noise of single-qubit quantum key distribution (QKD) protocols in the scenario of security against independent eavesdropping attacks and key distillation based on one-way classical communication. To this end, we introduce a noise-based metric that quantifies the efficiency of QKD protocols. Within this framework, we analyze the maximal noise levels that allow Alice and Bob to asymptotically establish a secure secret key. Using this assumption, we compare the noise tolerance of general single-qubit QKD protocols, in particular the BB84, B92, E91, and six-state protocols. Our main result determines the noise level threshold for QKD allowing one to distill an asymptotically secure secret key. Additionally, we demonstrate that the six-state protocol achieves the greatest resistance to noise while simultaneously yielding a higher post-selection efficiency than the other analyzed single-qubit protocols, confirming its robustness within the considered security model. Finally, we perform an analysis of the proposed noise-based metric and the conventional quantum bit error rate (QBER) metric.
Quantum Key Distribution (QKD) protocols offer top-tier secure communication but face challenges with noise, particularly when quantum and classical channels coexist. In fiber-optic systems, strong classical signals for data and synchronization create noise through scattering, with inelastic scattering being the main contributor. This broad-spectrum noise can overlap with quantum communication wavelengths, leading to errors and deterioration of the quantum signal. Our study examines the theoretical coexistence limit for a 1500 nm classical synchronization signal from a White Rabbit switch and a 1320 nm quantum signal from an S-Fifteen Instruments source, identifying the maximum fiber length where the quantum signal remains clear of noise. We chose the BB84 protocol for our QKD implementation, which we plan to first test in a laboratory setting without an intermediate node. In the future, adding a node (often referred to as Charlie) would improve scalability and help manage synchronization delays.
Twin-Field Quantum Key Distribution (QKD) is a QKD protocol that uses single-photon interference to perform QKD over long distances. QKD protocols that encode information using high-dimensional quantum states can benefit from increased key rates and higher noise resilience. We define the essence of Twin-Field QKD and explore its generalization to higher dimensions. Further, we show that, ultimately, the Twin-Field protocol cannot be generalized to higher dimensions in accordance with our definition.
Quantum key distribution protocols are easily the most advanced quantum information technology up to date, driving the whole field of quantum communications towards actual systems implementations and flourishing theoretical advances and experimental demonstrations. Exploiting the consolidated telecommunications industry knowledge, optimized equipment and already existing broad infrastructure, fiber-based quantum key distribution schemes are in particular a promising direction in which the scientific research efforts are addressed, for the final goal of building a quantum network. However, three main challenges are still preventing the large-scale industrial deployment of fiber-based quantum key distribution schemes: the limited achievable secret key generation rate, when compared to those reached by classical communications; the bounded propagation distances, due to intrinsic channel losses; and the struggle of the integration with classical data rates, which are additional sources of noise from the point of view of a quantum channel. During my Ph.D., we have focused on two of these problems, proposing new efficient designs for fiber-based high-dimensional protocols and space division multiplexing schemes to increase the secret key rate of quantum systems, and investigating the optimization of wavelength multiplexing setups for the integration with classical communications.
Security of the communications could be ensured using cryptography protocols. While asymmetrical protocols can be cracked using quantum computers, the symmetrical protocols stand against quantum attacks. However, the keys need to be exchanged in a secure way. A method is offered by the quantum key distribution (QKD) protocols. The QKD system should operate close to the fundamental quantum noise level. A possible attack is to steal some photons during the communication, but every change will result in some excess noise. This is why it is important to know the base noise level of the system, and every possible solutions need to be considered to reduce the noise of the system. To foster the research in this field, we started to develop a second generation QKD system over a 16 km long single mode, ordinary telecommunications fiber and focused on noise reduction.
Semiquantum key distribution protocols are designed to allow two parties to establish a shared secret key, secure against an all-powerful adversary, even when one of the users is restricted to measuring and preparing quantum states in one single basis. While interesting from a theoretical standpoint, these protocols have the disadvantage that a two-way quantum communication channel is necessary which generally limits their theoretical efficiency and noise tolerance. In this paper, we construct a semiquantum key distribution (SQKD) protocol which actually takes advantage of this necessary two-way channel, and, after performing an information theoretic security analysis against collective attacks, we show it is able to tolerate a channel noise level higher than any prior SQKD protocol to date. We also compare the noise tolerance of our protocol to other two-way fully quantum protocols, along with BB84 with classical advantage distillation. We also comment on some practical issues involving semiquantum key distribution (in particular, concerning the potential complexity in physical implementation of our protocol as compared with other standard QKD protocols). Finally, we develop techniques that can be applied to the security analysis of other (S)QKD protocols reliant on a two-way quantum communication channel.